This invention relates generally to apparatus for compensating for actuator rate saturation in servomechanisms, and more particularly to such apparatus for use with flight control actuators.
With the advent of more and more high performance jet aircraft, actuator rate saturation, wherein a phase lag develops between changes in command to the actuator and response from the aircraft, is becoming an increasingly critical issue in the flight controls arena. Actuator rate saturation can occur due to limitations in the hydraulic system or the physical limitations of the actuator itself.
In hydraulic system design there is a trade-off between high flow rate and high pressure. At high dynamic pressure (q), when airspeed is greatest, a very high hydraulic pressure is required to overcome the large hinge moments and effectively control the actuators. Under this circumstance, the actuator piston should be designed to have a large crosssectional area, so as to reduce the hydraulic pressure level required. At low dynamic pressure, a large flow rate is required for actuator control due to reduced control surface effectiveness at low speeds. The large flow is especially critical in the power approach situation. If an actuator is designed to have a large piston area for high q situations, low q conditions can only be satisfied with the actuator operation at a very high rate.
High performance jet aircraft, particularly short takeoff and landing (STOL) designs, must operate in both the high q/supersonic and low q/power approach situations. As a result of these conflicting demands on the actuator, rate saturation can occur, causing a phase lag problem, which in turn causes poor flight control performance and, often, pilot induced oscillations (PIOs). Another situation often occurs in which the pilot utilizes many of his control surfaces simultaneously. This results in reduced hydraulic pressure, which can cause actuator rate saturation due to the hydraulic limits of the actuator itself.